1. Field of the Invention
The present invention relates to a four-wheel drive vehicle in which a portion of a driving force from an engine is transmitted from an input shaft through a pair of bevel gears and left and right clutches to left and right wheels, and particularly, to a driving force control system in such a four-wheel drive vehicle.
2. Description of the Related Art
In such a four-wheel drive vehicle, an appropriate value of a maximum transmitted torque transmitted through the bevel gears and the clutches to the left and right wheels is varied depending upon the friction coefficient of a road surface. It is desirable that for a road surface of a higher friction coefficient such as an asphalt road, the maximum transmitted torque is set at a larger value, and for a road surface of a lower friction coefficient such as a snow-laden road, the maximum transmitted torque is set at a smaller value. It is required that the strength of the bevel gears and the capacity of the clutches are set such that they can withstand a maximum transmitted horsepower transmitted by the bevel gears and the clutches.
The maximum transmitted horsepower transmitted by the bevel gears and the clutches is proportional to the maximum transmitted torque and the vehicle speed. This causes a problem in that, if the maximum transmitted torque is of a given value, the maximum horsepower is increased with an increase in vehicle speed and hence, an increase in strength of the bevel gears and an increase in capacity of the clutches are required to bring about an increase in size of a power transmitting system.